1. Field of the Invention
This invention relates to methods of enabling the transmission and reception of signals through unshielded twisted pairs of wires within a communications system. The invention particularly relates to a startup protocol for initiating normal transmission between transceivers within a high throughput communications system.
A xe2x80x9chigh throughputxe2x80x9d as used within the context of this disclosure may include, but is not limited to, one gigabit (GB) per second.
2. Description of Related Art
A basic communications system is illustrated in FIG. 1. The system includes a hub and a plurality of computers serviced by the hub in a local area network (LAN). Four computers are shown by way of illustration but a different number of computers may be contained within the system. Each of the computers is usually displaced from the hub by a distance which may be as great as approximately one hundred meters (100 m.). The computers are also displaced from each other. The hub is connected to each of the computers by a communications line. Each communication line includes unshielded twisted pairs of wires or cables. Generally, the wires or cables are formed from copper. Four unshielded twisted pairs of wires are provided in each communication line between each computer and the hub. The system shown in FIG. 1 is operative with several categories of unshielded twisted pairs of cables designated as categories 3, 4 and 5 in the telecommunications industry. Category 3 cables are the poorest quality (and lowest cost) and category 5 cables are the best quality (and highest cost).
Associated with each communications system is a xe2x80x9cthroughputxe2x80x9d. The throughput of a system is the rate at which the system processes data and is usually expressed in bits/second. Most communications systems have throughputs of 10 megabits (Mb)/second or 100 Mb/second. A rapidly evolving area of communications system technology enables 1 Gb/second full-duplex communication over existing category-5 unshielded twisted pair cables. Such a system is commonly referred to as xe2x80x9cGigabit Ethernet.xe2x80x9d
A portion of a typical Gigabit Ethernet is shown in FIG. 2. The Gigabit Ethernet provides for transmission of digital signals between one of the computers and the hub and the reception of such signals at the other of the computer and the hub. A similar system can be provided for each of the computers The system includes a gigabit medium independent interface (GMII) block which receives data in byte-wide format at a specified rate, for example 125 MHz, and passes the data onto the physical coding sublayer (PCS) which performs scrambling, coding, and a variety of control functions. The PCS encodes bits from the GMII into 5-level pulse amplitude modulation (PAM) signals. The five signal levels are xe2x88x922, xe2x88x921, 0, +1, and +2. Communication between the computer and hub is achieved using four unshielded twisted pairs of wires or cables, each operating at 250 Mb/second, and eight transceivers, one positioned at each end of a unshielded twisted pair. The necessity of full-duplex bidirectional operation dictates the use of hybrid circuits at the two ends of each unshielded twisted pair. The hybrid controls access to the communication line, thereby allowing for full-duplex bidirectional operation between the transceivers at each end of the communications line.
A common problem associated with communications systems employing multiple unshielded twisted pairs and multiple transceivers is the introduction of crosstalk and echo noise or impairment signals into the transmission signals. Noise is inherent in all such communications systems regardless of the system throughput.
However, the effects of these impairment signals are magnified in Gigabit Ethernet. Impairment signals include echo, near-end crosstalk (NEXT), and far-end crosstalk NEXT) signals. As a result of these impairment signals the performance of the transceivers, particularly the receiver portion, is degraded.
NEXT is an impairment signal that results from capacitive coupling of the signals from the near-end transmitters to the input of the receivers. The NEXT impairment signals encountered by the receiver in transceiver A are shown in FIG. 3.
The crosstalk signals from transmitters B, C, and D appears as noise to receiver A, which is attempting to detect the direct signal from transmitter E. Each of the receivers in the system encounter the same effect and accordingly the signals passing through the receivers experience signal distortion due to NEXT impairment signals. For clarity of FIG. 3, only the NEXT impairment experienced by receiver A is illustrated.
Similarly, because of the bidirectional nature of the communications systems, an echo impairment signal is produced by each transmitter on the receiver contained within the same transceiver as the transmitter. The echo impairment signal encountered by the receiver in each transceiver is shown in FIG. 4. The crosstalk signals from transmitters appear as noise to the receivers, which are attempting to detect the signal from the transmitter at the opposite end of the communications line. Each of the receivers in the system encounter the same effect and accordingly the signals passing through the receivers experience signal distortion due to the echo impairment signal.
Far-end crosstalk (FEXT) is an impairment that results from capacitive coupling of the signal from the far-end transmitters to the input of the receivers. The FEXT impairment signals encountered by the receiver in transceiver A are shown in FIG. 5. The crosstalk signals from transmitters F, G, and H appears as noise to receiver A, which is attempting to detect the direct signal from transmitter E. Each of the receivers in the system encounter the same effect and accordingly the signals passing through the receivers experience signal distortion due to the FEXT impairment signal. For clarity of FIG. 5 only the FEXT impairment experienced by receiver A is illustrated.
Four transceivers at one end of a communications line are illustrated in detail in FIG. 6. The components of the transceivers are shown as overlapping blocks, with each layer corresponding to one of the transceivers. The GMII, PCS, and hybrid of FIG. 6 correspond to the GMII, PCS, and hybrid of FIG. 2 and are considered to be separate from the transceiver. The combination of the transceiver and hybrid forms one xe2x80x9cchannelxe2x80x9d of the communications system. Accordingly, FIG. 6 illustrates four channels, each of which operate in a similar manner. The transmitter portion of each transceiver includes a pulse-shaping filter and a digital-to-analog D/A) converter. The receiver portion of each transceiver includes an analog-to-digital (A/D) converter, a first-in first-out (FIFO) buffer, a digital adaptive equalizer system including a feed-forward equalizer (FIFE) and a detector. The receiver portion also includes a timing recovery system and a near-end noise reduction system including a NEXT cancellation system and an echo canceller.
One of the most critical phases of the operation of a Gigabit Ethernet transceiver is the startup. During this phase adaptive filters contained within the transceiver converge, the timing recovery subsystem acquires frequency and phase synchronization, the differences in delay among the four wire pairs are compensated, and pair identity and polarity is acquired. Successful completion of the startup allows normal operation of the transceiver to begin.
In one startup protocol, known as xe2x80x9cblind startxe2x80x9d, the transceivers converge their adaptive filters and timing recovery systems simultaneously while also acquiring timing synchronization. A disadvantage of such a startup is that there is a high level of interaction among the various adaptation and acquisition algorithms within the transceiver. This high level of interaction reduces the reliability of the convergence and synchronization operations which occur during startup.
Thus there exists a need in the art to provide a startup protocol for use in a high throughput communications system, such as a Gigabit Ethernet, that uses the optimal sequence of operations and minimizes the interaction among the various adaptation and acquisition algorithms. The present invention fulfills these needs.
Briefly, and in general terms, the invention relates to methods of enabling the transmission and reception of signals through unshielded twisted pairs of wires within a communications system. The invention particularly relates to a startup protocol for initiating normal transmission between transceivers within a high throughput communications system.
In one embodiment, the invention is a startup protocol for use in a communications system having a plurality of transceivers, one transceiver acting as a master and another transceiver acting as slave, each transceiver having a noise reduction system, a timing recovery system and at least one equalizer. The startup protocol includes the step of executing a first stage during which the timing recovery system and the equalizer of the slave are trained and the noise reduction system of the master is trained. Also included is the step of executing a second stage during which the timing recovery system of the master is trained in both frequency and phase, the equalizer of the master is trained and the noise reduction system of the slave is trained. Further included is the step of executing a third stage during which the noise reduction system of the master is retrained, the timing recovery system of the master is retrained in phase and the timing recovery system of the slave is retrained in both frequency and phase.
By separating the convergence of the equalizer and the timing recovery system from the convergence of the noise reduction system the interaction among the various adaptation and acquisition algorithms within the transceiver is reduced. As a result, the reliability of the convergence and synchronization operations which occur during startup is improved. In addition, by retraining the timing recovery system of the master in phase and the timing recovery system of the slave during the third stage, the protocol allows for the enabling of the transmission and reception of signals within a communications system having a xe2x80x9cfree-runningxe2x80x9d slave clock. A xe2x80x9cfree-runningxe2x80x9d slave clock is one which does not freeze or save its state variables once they have been acquired.
In an additional aspect of the first embodiment, the startup protocol further includes the steps of initiating the first stage, transitioning from the first stage to the second stage, and transitioning from the second stage to the third stage. In another facet the step of initiating the first stage comprises the steps of transmitting a signal from the master to the slave, detecting the signal at the slave, and if the slave is transmitting, ceasing transmission from the slave. In a further aspect of the invention, the step of transitioning from the first stage to the second stage comprises the steps of transmitting a signal from the slave to the master, detecting the signal at the master; and ceasing transmission from the master. In yet another aspect, the step of transitioning from the second stage to the third stage comprises the steps of transmitting a signal from the master to the slave, detecting the signal at the slave, and continuing transmission from the slave.
In a second embodiment, the invention is a startup protocol for use in a communications system having a master transceiver at one end of a communications line and a slave transceiver at the opposite end of the communications line, each transceiver having a near-end noise reduction system, a far-end noise reduction system, a timing recovery system and at least one equalizer. The protocol includes the step of, during a first phase, maintaining the master in a half-duplex mode during which it transmits a signal but does not receive any signals, maintaining the slave in a half-duplex mode during which it receives the signal from the master but does not transmit any signals, converging the master near-end noise reduction system, adjusting the frequency and phase of the signal received by the slave such that the frequency and phase are synchronized with the frequency and phase of the signal transmitted by the master, and converging the equalizer of the slave. The protocol further includes the step of, during a second phase, maintaining the slave in a half-duplex mode during which it transmits a signal using a free-running clock but does not receive any signals, maintaining the master in a half-duplex mode during which it receives the signal from the slave but does not transmit any signals, converging the slave near-end noise reduction system, adjusting the frequency and phase of the signal received by the master such that the frequency and phase are synchronized with the frequency and phase of the signal transmitted by the slave, and converging the equalizer of the master. The protocol also includes the step of, during a third phase, maintaining the slave in a full-duplex mode such that the slave transmits and receive signals, maintaining the master in a full-duplex mode such that the master transmits and receive signals, reconverging the master near-end noise reduction system, adjusting the phase of the signal received by the master such that the phase is synchronized with the phase of the signal transmitted by the slave, adjusting the frequency and phase of the signal received by the slave such that the frequency and phase are synchronized with the frequency and phase of the signal transmitted by the master.
In a third embodiment, the invention is a startup protocol for use in a communications system having a plurality of transceivers, a first one of the transceivers acting as a master and a second one of the transceivers acting as a slave, each of the first and second ones of the transceivers including a transmitter and a receiver. The protocol includes the step of initially operating, in a first phase, the first transceiver only as a transmitter and the second transceiver only as a receiver and adjusting the timing of the second transceiver in accordance with the timing of the first transceiver and minimizing a change in the operation of the first transceiver as a transmitter as a result of the operation of the first transceiver as a receiver. Also included is the step of operating, in a second phase, the first transceiver only as a receiver and the second transceiver only as a transmitter and adjusting the timing of the first transceiver in accordance with the timing of the second transceiver and minimizing a change in the operation of the second transceiver as a transmitter as a result of the operation of the second transceiver as a receiver. Also included is the step of operating, in a third phase, the first and second transceivers simultaneously as transmitters and receivers, to adjust and fix the change in the operation of the first transceiver as a receiver as a result of the operation of the first transceiver as a transmitter.
In a fourth embodiment, the invention is a startup protocol for use in a communications system having plurality of transceivers, a first one of the transceivers acting as a master and a second one of the transceivers acting as a slave, each transceiver including a transmitter and a receiver. The protocol comprises the step of initially operating each of the first and second transceivers only as a transmitter and the other of the first and second transceivers only as a receiver to minimize a change in the operation of the transmitting transceiver transmitter as a result of the operation of the transmitting transceiver receiver and to provide adjustments in the timing of the receiving transceiver in accordance with the timing of the transmitting transceiver. Also included is the step of operating each of the first and second transceivers simultaneously both as a transmitter and a receiver to minimize a change in the operation of the transmitter in the first transceiver as a result of the operation of the receiver in the first transceiver and to provide adjustments in the timing of each transceiver in accordance with the timing of the other transceiver.
These and other aspects and advantages of the present invention will become apparent from the following more detailed description, when taken in conjunction with the accompanying drawings which illustrate, by way of example, the preferred embodiments of the invention.